Field of the Invention
The present invention relates to a switching power supply, and more particularly, to a switching power supply with a preventive maintenance function to detect a power supply operation destabilized before a reduction in output voltage.
Description of the Related Art
A switching power supply outputs a constant voltage mainly by pulse width modulation (PWM) control. The following is a description of some prior art examples for detecting an abnormality in a power supply circuit comprising the switching power supply of this type.
First Prior Art Example
For example, Japanese Patent Applications Laid-Open Nos. 02-145977 and 2005-172653 disclose how to detect an abnormality in a power supply circuit including a switching power supply by an output voltage monitoring circuit incorporated in the power supply circuit. According to this prior art technique, as shown in FIG. 4, an output voltage is monitored by the voltage monitoring circuit in the power supply so that an abnormality in the power supply can be systematically recognized.
Second Prior Art Example
For example, Japanese Patent Application Laid-Open No. 05-056629 discloses a technique for estimating a remaining life by applying an operation status to an estimation model for estimating the degree of progress of degradation (life).
Third Prior Art Example
Japanese Patent Applications Laid-Open Nos. 2002-153064 and 2001-275344 disclose a switching power supply circuit configured so that an abnormality in the power supply circuit is detected if a value (such as a switching frequency) obtained by monitoring a switching waveform of a switching power supply deviates from a predetermined threshold range.
However, the above-described prior art examples have the following problems.
In the first prior art example, if the output voltage deviates from the monitored voltage due to aging degradation of an output capacitor or the like, the abnormality can be detected by the power supply circuit comprising the voltage monitoring circuit. If the threshold of the monitored voltage is low, however, a period from the detection of the abnormality to the occurrence of a failure is so short, as shown in FIG. 5, that the operation may be suddenly interrupted if preparation of a replacement part is delayed.
If the threshold of the monitored voltage is increased more than necessary, a normal operation period becomes so short, as shown in FIG. 5, that the costs increase as a side effect, though the necessary period from the detection of the abnormality to the occurrence of the failure can be secured for the preparation of the replacement part or the like.
There is also a problem that the voltage monitoring IC in the power supply circuit performs monitoring on a DC level, so that it is necessary to separately provide a structure for monitoring a ripple voltage that influences the power supply precision.
In the second prior art example, as a model reflective of an actual circuit state is not used, this technique is applicable to an situation where an estimation model is applicable, but in a case where a degradation mode different from the estimation model is caused, a failure occurs during a period shorter than the estimated remaining life, resulting in a sudden interruption of the operation.
In the third prior art example, moreover, the switching power supply of an insulating AC-DC converter is assumed to be an object of switching frequency monitoring. Since this insulating AC-DC converter is a high-power application, components are quickly degraded. If the operation (characteristic) of the switching power supply after occurrence of abnormality in the switching frequency is not set, as in this prior art example, it is difficult to set a monitoring threshold with a period from the detection of the abnormality in the switching frequency to the occurrence of an output power supply abnormality properly secured. Thus, an advantageous effect of preventive maintenance of this example is low.